When the bathroom toilet is flushed, the water level is higher than it should be, there is spluttering of water coming from the bath plug hole and knocking sounds from the air admittance valve fitted below the hand wash basin. Water draining from the basin also sometimes causes spluttering in the bath. The bathroom toilet water level returns to normal. But the downstairs toilet is not flushing properly and the water level remains high. Before this the level has been also very low. The symptoms are puzzling and variable.